In a conventional seat structure for a vehicle or especially for a van, a seat on which two passengers can sit side by side and a seat on which one passenger sits have a 6:4 relation in size. Each of these seats can slide in a front-rear direction by use of slide rails arranged side by side on a floor. Further, safety belts are arranged for the two-passenger seat and the one-passenger seat to allow a total of three passengers to be strapped. One base end of each safety belt is fixed to a frame of a vehicle body or of the seat. Mother base end of the safety belt is attached to the corresponding slide rail so as to be able to change in position in the front-rear direction along with the motion of the seat (see Patent Document 1). Thus, the safety belt can be fastened regardless of where the seat is positioned in the front-rear direction.